The cyclins are a family of cell cycle-related proteins that are expressed at different times and are involved in regulation of cells through the various phases of the cell cycle. In general, cyclins D and E appear primarily during G1 phase, cyclin A during S and cyclin B in G2 phase. The cyclins complex with cyclin-dependent kinases and these complexes phosphorylate proteins that are important for traverse of cells through the cell cycle. Antibodies are now available to the major cyclins and utilization of these probes for analysis of cyclin expression provides an important tool for measuring the position and frequency of cells in the various phases of the cell cycle. Darzynkiewicz and co-workers have developed and utilized cell-labeling and FCM techniques to demonstrate the correlation of cyclin D and E expression and staurosporine-induced G1 arrest of human lymphocytes and the effects of G2 arrest and polyploidization of transformed cells and cyclin B. Utilizing similar method we are investigating the expression of cyclins on human diploid fibroblasts following exposure to alpha and gamma irradiation.